1. Field of the Invention
This invention relates to an integrated circuit formed upon a frontside surfaces of semiconductor substrates, and more particularly to failure analysis and fault isolation techniques employed in cases where the frontside surfaces are inaccessible, or where several layers of metal interconnects prevent the use of more conventional circuit probing techniques upon the frontside surface.
2. Description of the Relevant Art
During manufacture of an integrated circuit (e.g., a microprocessor), electronic components are formed upon and within a frontside surface of a semiconductor substrate having opposed frontside and backside surfaces. The electronic components are connected together by electrically conductive interconnect (i.e., signal) lines, forming an electronic circuit. Signal lines which are to be connected to external devices are terminated at flat metal contact regions called input/output (I/O) pads. Following manufacture, the integrated circuit (i.e., "chip") is typically secured within a protective semiconductor device package. Each I/O pad of the chip is then connected to one or more terminals of the device package. The terminals of a device package are typically arranged about the periphery of the package. The I/O pads of the chip are electrically connected to the terminals of the device package. Some types of device packages have terminals called "pins" for insertion into holes in a printed circuit board (PCB). Other types of device packages have terminals called "leads" for attachment to flat metal contact regions on an exposed surface of a PCB.
As integrated circuit fabrication technology improves, manufacturers are able to integrate more and more functions onto single silicon substrates. As the number of functions on a single chip increases, however, the number of signal lines which need to be connected to external devices also increases. The corresponding numbers of required I/O pads and device package terminals increase as well, as do the complexities and costs of the device packages. Constraints of high-volume PCB assembly operations place lower limits on the physical dimensions of and distances between device package terminals. As a result, the areas of peripheral-terminal device packages having hundreds of terminals are largely proportional to the number of terminals. These larger packages with fine-pitch leads are subject to mechanical damage during handling or testing. Mishandling can result in a loss of lead coplanarity, adversely affecting PCB assembly yields. In addition, the lengths of signal lines from chip I/O pads to device package terminals increase with the number of terminals, and the high-frequency electrical performance of larger peripheral-terminal device packages suffer as a result.
Grid array semiconductor device packages have terminals arranged in a two-dimensional array across an underside surface of the device package. As a result, the physical dimensions of grid array device packages having hundreds of terminals are much smaller than their peripheral-terminal counterparts. Such smaller packages are highly desirable in portable device applications such as laptop and palmtop computers and handheld communications devices such as cellular telephones. In addition, the lengths of signal lines from chip I/O pads to device package terminals are shorter, thus the high-frequency electrical performances of grid array device packages are typically better than those of corresponding peripheral-terminal device packages. Grid array device packages also allow the continued use of existing PCB assembly equipment developed for peripheral-terminal devices.
An increasingly popular type of grid array device package is the ball grid array ("BGA") device package. FIG. 1 is a cross-sectional view of an exemplary ball grid array (BGA) device 10 including an integrated circuit 12 mounted upon a larger package substrate 14. Substrate 14 includes two sets of bonding pads: a first set of bonding pads 16 on an upper surface adjacent to integrated circuit 12 and a second set of bonding pads 18 arranged in a two-dimensional array across an underside surface. Integrated circuit 12 includes a semiconductor substrate 20 having multiple electronic components formed within a circuit layer 22 upon a frontside surface of semiconductor substrate 20 during wafer fabrication. The electronic components are connected by electrically conductive interconnect lines, forming an electronic circuit. Multiple input/output (I/O pads) 24 are also formed within circuit layer 22. I/O pads 24 are typically coated with solder, forming solder "bumps" 26.
The integrated circuit is attached to the package substrate using the controlled collapse chip connection (C4.RTM. or "flip chip") method. During the C4.RTM. mounting operation, solder bumps 26 are placed in physical contact with corresponding members of the first set of bonding pads 16. Solder bumps 26 are then heated long enough for the solder to reflow. When the solder cools, I/O pads 24 of integrated circuit 12 are electrically and mechanically coupled to the corresponding members of the first set of bonding pads 16 of the package substrate. After integrated circuit 12 is attached to package substrate 14, the region between integrated circuit 12 and package substrate 14 is filled with an "underfill" material 28 which encapsulates the C4.RTM. connections and provides other mechanical advantages.
Package substrate 14 may be made of, for example, fiberglass-epoxy printed circuit board material or ceramic material (e.g., aluminum oxide, alumina, Al.sub.2 O.sub.3, or aluminum nitride, AlN). Package substrate 14 includes one or more layers of signal lines (i.e., interconnects) which connect respective members of the first set of bonding pads 16 and the second set of bonding pads 18. Members of the second set of bonding pads 18 function as device package terminals and are coated with solder, forming solder balls 30 on the underside surface of package substrate 14. Solder balls 30 allow BGA device 10 to be surface mounted to an ordinary PCB. During PCB assembly, BGA device 10 is attached to the PCB by reflow of solder balls 30 just as the integrated circuit is attached to the package substrate.
The C4.RTM. mounting of integrated circuit 12 to package substrate 14 prevents physical access to circuit layer 22 for failure analysis and fault isolation. However, several analytic and diagnostic techniques developed to reveal defects and logic states within integrated circuits are also useful when applied to flip chip grid array devices. Some of these techniques involve stimulation of a target portion of circuit layer 22 with electromagnetic radiation. For example, silicon substrates transmit a significant fraction of incident laser light having wavelengths from about 1,000 nanometers (nm) to upwards of 1,800 nm. Photons of laser light with wavelengths from about 1,000 nm to approximately 1,200 nm have sufficient energy to create electron-hole pairs in some silicon substrates used for wafer fabrication when absorbed during collisions with atoms of elements within the silicon substrates. The electrons and holes (i.e., charge carriers) thus created cause detectable changes in (i.e., stimulate) an isolated target portion of circuit layer 22. Photons of laser light having wavelengths greater than or equal to about 1,300 nm lack sufficient energy to create electron-hole pairs during collisions. However, the magnitude and/or phase of a reflected portion of an incident laser beam having a wavelength of about 1,300 or greater is affected by electric fields and charge modulation effects existing within circuit layer 22. Techniques which detect the reflected portion allow imaging within the silicon substrates.
FIG. 2 is a cross-sectional view of integrated circuit 12 of BGA device 10 of FIG. 1 undergoing such electromagnetic stimulation. A beam of laser light (i.e., an incident beam 32) having a sufficient wavelength is directed onto an upward-facing backside surface 34 of integrated circuit 12. A portion of the incident beam (i.e., a reflected portion 36) is reflected from backside surface 34, and the remainder (i.e., a transmitted portion 38) is transmitted into semiconductor substrate 20. Photons of transmitted portion 38 are absorbed within semiconductor substrate 20 during collisions with atoms of elements within semiconductor substrate 20. The electrons and holes created when the photons are absorbed near target portion 40 of circuit layer 22 stimulate the electronic components within target portion 40. Some analytic methods detect changes in the amount of electrical power delivered to integrated circuit 12 subject to such electromagnetic stimulation. For example, U.S. Pat. No. 5,430,305 describes the scanning of an incident laser beam across a backside surface of an integrated circuit and the detection of resultant changes in the voltage of a constant-current power supply delivering electrical power to the integrated circuit in order to image or "map" logic states of electronic components within the circuit layer.
When photons of transmitted portion 38 strike electronic components or encounter electric fields within target portion 40, part of transmitted portion 38 is reflected back toward backside surface 34. At the interface between semiconductor substrate 20 and the surrounding air, part of the light emanating from target portion 40 is reflected back into semiconductor substrate 20, and the remainder emerges from the backside surface as an exit beam 42. Scanning laser beam microscopes scan incident beam 32 across a surface of a specimen (e.g., semiconductor substrate 20) and detect the intensity of exit beam 42 emerging from the surface, allowing imaging of structures within the specimen.
FIG. 3 is a cross-sectional view of integrated circuit 12 of the BGA device 10 of FIG. 1 illustrating another diagnostic technique useful in revealing defects within integrated circuit 12. Electron-hole pairs present within semiconductor substrate 20 recombine at crystalline defect sites (i.e., impurities, dislocations, stacking faults, etc.) within semiconductor substrate 20. When the semiconductor material of substrate 20 is silicon, a small portion of electron-hole pairs emit a photon of light (i.e., electromagnetic radiation) when they recombine. Other types of defects such as thin metal-to-metal "filaments" also emit electromagnetic radiation.
A relatively large component of the electromagnetic radiation emitted by defects exists in the near infrared region of the electromagnetic spectrum. Defects within integrated circuit 12 may thus be located by detecting near infrared radiation (i.e., light) 46 emitted from within a portion 48 of circuit layer 22 during operation of integrated circuit 12. At the interface between semiconductor substrate 20 and the surrounding air, a portion of the emitted light is reflected back into semiconductor substrate 20, and the remainder emerges from backside surface 34 as an exit beam 50. Detection of exit beam 50 allows the locations of certain types of defects within the semiconductor substrate 20 to be determined. Emission microscopes employ this technique.
Several problems arise when employing the above analytic and diagnostic techniques which rely upon either electromagnetic stimulation of electronic components within circuit layer 22 (FIG. 2) or the detection of light emitted at defect sites within circuit layer 22 (FIG. 3). In FIG. 2, a large fraction of incident beam 32 is reflected at backside surface 34 (i.e., the intensity of reflected portion 36 is a substantial portion of the intensity of incident beam 32). As a result, the intensity of transmitted portion 38 is relatively low, reducing the level of stimulation of electronic components within target portion 40 of circuit layer 22. Due to the low level of stimulation, the responses from electronic components within target portion 40 have low magnitudes. Such low magnitude responses may be difficult to detect over background "noise" levels, reducing the resolutions of scanned images. In addition, where detection of exit beam 42 is relied upon to create a scanned image, reflected portion 36 and stray exit beams (e.g., stray exit beam 44) caused by reflections within semiconductor substrate 20 represent substantial background noise levels. In FIG. 3, stray exit beams (e.g., stray exit beam 52) also contribute to the background noise level which makes detection of relatively low light levels emanating from within portion 48 of circuit layer 22 difficult and reduces the resolutions of scanned images.
It would be beneficial to have various methods for analyzing an integrated circuit from the backside surface which employ measures to reduce: (i) the large fraction of an incident beam reflected from the backside surface when the incident beam strikes the backside surface, and/or (ii) stray exit beams caused by reflections within the semiconductor substrate. Reducing such reflections would lower background noise levels, thereby increasing the detection capabilities of the methods and the resolutions of resulting scanned images.